Periodic breathing refers to abnormal respiration patterns that alternate between hypopnea (i.e. diminished breathing) and hyperpnea (i.e. fast, deep breathing.) One form of periodic breathing is Cheyne-Stokes Respiration (CSR), which can occur in patients with congestive heart failure (CHF). Briefly, CSR arises principally due to a time lag between blood carbon dioxide (CO2) levels sensed by the central nervous system and the blood CO2 levels. With CHF, poor cardiac function results in poor blood flow to the brain such that the central nervous system responds to blood CO2 levels that are no longer properly representative of the overall blood CO2 levels in the body. Hence, the central nervous system triggers an increase in the depth and frequency of breathing in an attempt to compensate for perceived high blood CO2 levels whereas the blood CO2 levels have already dropped. By the time the central nervous system detects the drop in blood CO2 levels and slows respiration in response, the blood CO2 levels have already increased. This cycle becomes increasingly unbalanced until respiration periodically alternates between hypopnea (or hypoventilation) and hyperpnea (or hyperventilation). The wildly fluctuating blood chemistry levels can exacerbate CHF and other medical conditions. Moreover, the periods of hypopnea can be sufficiently pronounced that no breathing occurs, i.e. the patient suffers from episodes of frank apnea. The episodes of apnea can last so long that the patient, if sleeping, is awakened due to increasingly high blood CO2 levels. Arousal from sleep usually lasts only a few seconds, but even brief arousals nevertheless disrupt continuous sleep and can prevent the patient from achieving rapid eye movement (REM) sleep, which is needed.
Another form of periodic breathing can arise due to central sleep apnea (CSA), which is a neurogenic sleep disorder. When blood CO2 levels exceed a certain threshold, the central nervous system should generate a burst of nerve signals for triggering inspiration. The nerve signals are relayed via phrenic nerves to the diaphragm and via other nerves to chest wall muscles, which collectively contract to expand the lungs. With CSA, however, the nerve signals are not properly generated at times while the patient is asleep or are of insufficient magnitude to trigger sufficient muscle contraction to achieve inhalation. In either case, the patient thereby fails to inhale until appropriate respiratory nerve signals are eventually generated—at which point fast, deep, rapid breathing occurs (i.e. hyperpnea) to compensate for the increased blood CO2 levels arising due to the episode of CSA. In some cases, the episodes of CSA are fairly periodic and so periods of apnea alternate with periods of hyperpnea. In other words, a form of periodic breathing similar to CSR occurs. Note that, in the literature, CSR is sometimes classified as a type of CSA, regardless of the cause of the CSR. Herein, however, the term CSA is used to refer to the above-described neurogenic sleep disorder, which may or may not trigger periodic breathing.
In view of the significant adverse consequences of periodic breathing, particularly insofar as patients with CHF are concerned, it is highly desirable to provide techniques for detecting treating periodic breathing. Heretofore, periodic breathing therapy has been directed to improving respiration during the hypopnea/apnea phase. This may be achieved by, for example, applying diaphragmatic stimulation via phrenic nerve stimulation during the hypopnea/apnea phase using an implantable nerve stimulation system. The implantable nerve stimulation system may utilize a pacemaker or ICD as a controller to coordinate the detection of periodic breathing and the delivery of stimulation therapy in response thereto. Pacemakers and ICDs are usually implanted primarily for use in applying cardiac therapy for treating arrhythmias or for delivering cardiac resynchronization therapy (CRT) in an effort to alleviate CHF. However, many patients who are candidates for pacemakers or ICDs also suffer from CSR and hence could benefit from additional functionality directed to the detection and treatment of periodic breathing. Indeed, since periodic breathing can exacerbate CHF—yielding a higher risk of stroke or heart attack—CHF patients who would otherwise have pacemakers implanted therein could significantly benefit from periodic breathing therapy as well. An example of a technique for performing diaphragmatic stimulation during the hypopnea/apnea phase of periodic breathing using an implantable medical system incorporating a pacemaker is set forth in U.S. Pat. No. 6,415,183 to Scheiner et al., entitled “Method and Apparatus for Diaphragmatic stimulation.”
With conventional diaphragmatic stimulation techniques, care should be taken to ensure that stimulation provided during the hypopnea/apnea phase does not inadvertently induce an upper airway occlusion due to decreased intrathoracic pressure caused by the diaphragmatic stimulation (i.e. decrease pressure relative to external air pressure.) In this regard, during normal respiration, the central nervous system provides stimulation signals to the phrenic nerves for contracting the diaphragm to induce respiration and simultaneously provides stimulation signals to muscles in the upper airway surrounding the respiration airway. Stimulation is provided by the central nervous system to increase muscle tone in the upper airway during inspiration by an amount to prevent collapse of the upper airway. However, with implantable stimulation systems, artificial stimulation may potentially be applied only to the phrenic nerves and not to the muscles surrounding the upper airway and so there is a risk that the upper airway will collapse during the artificially-induced respiration. This problem may be addressed by implanting additional stimulation devices for directly stimulating the muscles of the throat at the same time that the phrenic nerves are stimulated. Techniques are described in U.S. patent application Ser. No. 10/795,009 of Koh et al., entitled “System And Method For Distinguishing Among Obstructive Sleep Apnea, Central Sleep Apnea And Normal Sleep Using An Implantable Medical System”, filed Mar. 3, 2004.
Although techniques such as Koh's for performing diaphragmatic stimulation during the hypopnea/apnea phase of periodic breathing are promising, it would be desirable to provide alternative techniques for treating periodic breathing using an implantable stimulation system and it is to that end that the present invention is primarily directed.